Known lasers, and other such emitters, have generally suffered from being capable of producing energy output only at very specific frequencies and wavelengths. The reason for this is best explained with a short discussion from the field of physics known as quantum mechanics.
Quantum mechanics tells us that electrons in materials can exist at different energy levels (states). When an electron makes a transition from a higher energy level to a lower energy level, radiation energy is emitted. Conversely, when radiation is absorbed the electron makes a transition from a lower energy level to a higher energy level. In each instance, the transition energy (i.e., the difference between the two energy levels) dictates the wavelength and frequency of the light radiated or absorbed.
In conventional semiconductor lasers, electrons make a transition from an energy level near the conduction band edge (an energy level at which the electrons can propagate through the material to result in electrical conduction) to an energy level near the valence band edge (a lower energy level). The net difference between these two levels varies with the host material. As a consequence, in order to change the frequency and wavelength of known lasers, one must use a different material, which all too often points to using undesirable or hazardous materials. Further, while there is a specific frequency (and thus wavelength) for each material, there is not a material for every frequency. Thus, the frequencies available to the designers and users of lasers have been rather limited. Moreover, for each material the transition occurs only from a very narrow range of higher energy levels to a very narrow range of lower energy levels, and vice versa. The implication of this is that for a laser of a particular material, only a very narrow band of frequencies and wavelengths can be produced.
Semiconductor fabrication techniques have advanced to the point that it is now possible to construct semiconductor structures made up of exceedingly thin layers, the layers being as thin as only a few atoms thick. Such precise fabrication capabilities have allowed for the investigation and development of so-called "quantum wells" in semiconductor structures. In a simple quantum well structure, a narrow band gap semiconductor layer is surrounded by regions of wide band gap semiconductor. A potential energy graph for such a structure resembles a deep, narrow trough, hence the name quantum "well". Quantum well lasers have been produced based on optical transitions from bound electron energy levels in the conduction band quantum well to bound electron energy levels in the corresponding valence band quantum well.
However, a quantum well can also be formed by two finite-thickness potential energy barriers (thin layers of electron high potential energy material) positioned on opposite sides of an electron low potential energy material. Given two energy states (levels) in a quantum well formed by two finite-thickness potential barriers, elementary quantum mechanics tells us that the lower energy state is more tightly bound than the upper state. This produces a longer electronic lifetime in the lower state than in the upper state (i.e., the electrons tend to stay in the lower state longer than in the upper state). This ratio of lifetimes is opposite that needed for laser action between these states. Further, the lifetime of the lower energy state must be significantly shorter than the electron scattering time in order to be able to deplete the electrons from the well. This fast depletion is also required for laser action. These facts have blocked the development of general-purpose lasers based on these transitions in a quantum well semiconductor structure.
A "quasibound" electron energy level in a multilayer semiconductor quantum device is an electron energy state that lies above the conduction band edges of the materials surrounding the quantum device structure. Electrons at these energies are traveling waves. These energy states are characterized by mathematically complex energy eigenvalues. By contrast, a "bound" electron energy level in a multilayer semiconductor quantum device is an electron energy level that lies below the conduction band edges of the materials surrounding the quantum device structure. Electrons at these energies are standing waves. These energy states are characterized by mathematically real energy eigenvalues. Quasibound hole energy levels are similarly defined below the valence band edge. A quasibound level may be above all conduction band edges of the multilayer quantum structure. A quasibound level may be in the conduction band and below one or more conduction band edges of the multilayer structure. Similarly, a quasibound level may be in the valence band below all valence band edges of the multilayer quantum structure or may be in the valence band above one or more valence band edges of the multilayer structure. In the literature, some quasibound electron states are also called "extended" states or "virtual" states if they lie above all conduction band edges in the multilayer structure. Quasibound states are analogous to leaky-guided modes in electromagnetic waveguides.
In 1971, investigators Kazarinov and Suris first proposed the use of semiconductor superlattices for infrared emitters. Kazarinov, R. F. & Suds, K. A. Sov. Phys. Semicond. 5, 707-709 (1971). In 1984, researchers West and Eglash observed large dipole infrared transitions in semiconductor quantum wells. West, L. C. & Eglash, S. J. Appl. Phys. Lett. 46, 1156-1158 (1985). Since then, a variety of structures have been proposed for intersubband emitters and lasers. Spontaneous emission between two quantum well levels has been observed in a GaAs/GaA1As structure. However, no intersubband laser has been developed at the present time. This is primarily due to the difficulty in obtaining "population inversion" between the subband states in a quantum well. Population inversion requires the upper energy level to be more tightly bound than the lower energy level. Recall from the discussion above that the opposite is the usual case. Previously proposed structures have relied on electron tunneling for admission into the upper energy state and for depletion from the lower energy state.
Investigators Helm and Allen have shown that it is difficult to achieve population inversion when a tunneling scheme is used. Helm, M. & Allen, S. J. Appl. Phys. Lett. 56, 1368-1370 (1990). An example of a prior art electron tunneling structure is shown in FIG. 1. Generally, electron tunneling can occur across a thin potential barrier from one side of this barrier where a large number of electrons are available to a large number of empty states on the other side of the barrier.
In 1977 researchers Hertick and Stillinger proposed the existence of above-barrier quasibound states in semiconductor heterostructures. Herrick, D. R. Physica B 85, 44-50 (1977). Stillinger, F. H. Physica B 85, 270-276 (1977). Recently, some of the present inventors have developed exact quantitative analogies between electron wave propagation in semiconductors and electromagnetic wave propagation in dielectrics. Henderson, G. N, Gaylord, T. K. & Glytsis, E. N. Proc. IEEE 79, 1643-1659 (1991). Simultaneously, some of the present inventors have developed a quantitative procedure for designing electronwave Fabry-Perot interference filters using standard thin film optical techniques. Gaylord, T. K. & Brennan, K. F.: Appl. Phys. Lett. 53, 2047-2049 (1988); J. Appl. Phys. 65, 2535-2540 (1989); J. Appl. Phys. 66, 6158-6167 (1989).
From the above, it can be seen that a need yet remains for a new class of emitters, detectors and other devices which utilize optical transitions between quasibound states and which are not limited to only certain specific wavelengths and frequencies. It is to the provision of such devices that the present invention is primarily directed.